Epoxy compositions are known in the industry and have been used for various applications such as paints, coatings, and adhesives. Epoxy adhesives are especially useful as structural adhesives for bonding metal surfaces together. Structural epoxy adhesives have replaced spot welding and other methods of mechanical fastening in many industrial assembly processes. In many applications, the surface of the metal substrate is contaminated with a mill oil, a cutting fluid, a draw oil, or a combination of the aforementioned fluids. The oily metal surface may further be contaminated with dirt or dust commonly found in many factories or warehouses.
To obtain a strong structural bond with an adhesive, it is often necessary to clean or degrease the metal using organic solvents. The use of such solvents in the workplace is highly undesirable because of environmental damage as well as industrial hygiene concerns.
Adhesives are available which bond to oily metal surfaces. The state of the art adhesives provide adequate bonding when used in a situation where the bonded substrate is exposed to heat immediately. However, in some situations, the substrate is left at room temperature for a period of time before curing at an elevated temperature. In these situations, the adhesives generally do not build as high of a shear strength as desired. Additionally, the failure mode in these situations is an adhesive failure wherein the adhesive pulls cleanly away from one of the substrates, indicating poor adhesion. It is generally desirable to have structural adhesives fail in a cohesive mode wherein the adhesive splits and portions of the adhesive remain adhered to each of the bonded surfaces. A bond that fails cohesively at high shear values is referred to as being "robust".
One such situation exists in automotive assembly plants. Adhesives used to hold metal automobile panels together are spot cured by induction heating in several places to hold the panels in place, but a significant portion of the adhesive is left uncured at ambient temperature (approximately room temperature) until the vehicle is painted and run through a paint bake cycle to cure the paint and the adhesive. The vehicle can be left at ambient temperature for any amount of time from several minutes up to several days, depending upon when the vehicle is run through the paint bake cycle.
Various attempts have been made to formulate epoxy adhesives for oily metal surfaces. Co-pending U.S. application Ser. No. 07/900,607 describes an epoxy adhesive for oily metals. In this application, the epoxy adhesive is a two-part thermally curable composition having an epoxy resin, a substituted pentafluoroantimonic acid, a substituted aniline, a polyol, and a toughening agent. While having its own utility and specific benefits, the adhesive has not been seen to exhibit robust cohesive failures when cured partially at room temperature and fully cured in an oven.
U.S. Pat. No. 4,920,182 describes an epoxy adhesive composition having an epoxy resin, a radiation activated catalyst, and a flexible polyester having terminal carboxyl groups. The polyester is formed from an aliphatic polyol, cycloaliphatic polyol, or a mixture of such polyols, and an aliphatic, cycloaliphatic, or aromatic polycarboxylic acids, or a mixture of such acids, or the anhydride of an aliphatic dicarboxylic acid. The composition is used for bonding etched aluminum.
An epoxy adhesive composition that is capable of forming a durable, robust bond on oily metal with a delayed thermal cure is desired.